Circuit design is big business and its design implementation has been trickling down to the maker market since the onset of single-board computers, which have become increasingly popular in recent years. This is also prevalent in the microcontroller (MCU) market that allows customers to configure the system to suit their needs. Of course, most of the companies that manufacture MCUs have their own set of free tools customers can use to help them along, such as Microchip’s MPLAB development platform. This platform includes everything from compilers to emulators and debuggers for just about every facet of circuit design. Some of the more popular tools include the MPLAB X IDE for developing microcontroller/digital signal controller (DSC) code, MPLAB ICD 3 for in-circuit debugging and MPLAB REAL ICE emulator for Microchip Flash DSC and MCU devices.

Quite Universal Circuit Simulator (QUCS) is a great little Spice-like open-source tool that runs quickly, has a wide range of components, rather intuitie interface (but a so-so manual), and will execute S-parameter analysis. Available at qucs.sourceforge.net/.